I always attempt to request a window seat whenever flying for many reasons, especially since I usually fly economy class
I can enjoy the scenery of my homeland or a new destination from a unique, omniscient perspective.
He who inhabits the window seat is the one who controls the blind. Some inconsiderate people may choose to leave the blind up as they read (or sleep), filling the cabin with light while the in-flight movie is on.
Possession of a window or aisle seat means the likelihood that you will be seated next to a malodorous voluble obese lunatic are halved.
Nobody will disturb you if they need to go to the toilet. On the other hand, you are likely to disturb others when you attend to your own ablutions.
Likewise, you will be less likely of being scalded if the flight attendant is pouring coffee for your neighbour when the plane hits an air pocket.
You are not only guaranteed one arm rest for your exclusive use, but you can form the most comfortable substitute for a bed using your seat. Recline your seat, stuff the cavity between the top part of your reclined seat and the wall with pillows, and lie back.
By default, after any seating preferences have been assigned, passengers are allocated first to window seats (equally on either side of the aircraft, and progressively down towards the rear). Aisle seats are then issued, and those who come last to the check-in can end up stuck between two people.
The probability of getting a window seat in economy class are at least:
On an Airbus 340/330: 25% (two seats in every row of eight seats)
On a Boeing 747: 20% (two seats in every row of ten seats)
On a Boeing 777: 22.2% (two seats in every row of nine seats)
On a Boeing 767: 28.6% (two seats in every row of seven seats)
On an Airbus 320/310 or Boeing 737: 33% (two seats in every row of six seats)
Note that in most aircraft configurations there are fewer seats per row at the rear.